Subwavelength optical microstructure light-redirecting films

ABSTRACT

A waveguide for use in a back lighting device includes a moth-eye structured surface. A back lighting system includes a lighting device, a display panel, and a waveguide for redirecting light generated by the light source toward the display panel. A first light-redirecting film, which can include a plurality of moth-eye structures, can be positioned between the waveguide and the display panel. The first light-redirecting film can also include a plurality of linear prisms. A second light-redirecting film is also provided that can include a plurality of moth-eye structures and a plurality of linear prisms. The second light-redirecting film can be positioned between the first light-redirecting film and the display panel. A diffuser can be positioned between the first light-redirecting film and the waveguide and/or between the second light-redirecting film and the display panel.

RELATED APPLICATIONS

[0001] This application is a Continuation-in-Part Application of U.S.patent application Ser. No. 09/684,455, filed Oct. 6, 2000, which is aContinuation-in-Part Application of U.S. patent application Ser. No.09/438,912, filed Nov. 12, 1999 (now U.S. Pat. No. 6,356,389). Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] Brightness enhancing films (BEF) have been used in lightingpanels for directing light from lighting fixtures through luminaires andlaptop computers displays. The brightness enhancing films, which canhave linear prisms, diffuse light with a desired directionality. Oftenthe films have been used in combination with a fluorescent light source.The films have had partial success in improving luminaire or displaybrightness by controlling the angle at which light emerges. However, aneed still exists for improved control of lighting and enhancement ofbrightness for laptop computer screens.

SUMMARY OF THE INVENTION

[0003] The present invention includes a light-redirecting or collimatingfilm having a sheeting having a first side and a second side, whereinthe first side includes a series of linear optical elements having aprimary axis running the length of the optical elements, and the secondside includes a plurality of subwavelength structures being oriented atabout 90 degrees relative to the primary axis of the linear opticalelements. The subwavelength structures can include linear moth-eyestructures. In one embodiment, the linear optical elements are linearprisms having an included angle in the range of between about 60 and 120degrees. In another embodiment, the linear optical elements includelenticular linear elements. In a particular embodiment, the prisms havean included angle of about 88 degrees. In another particular embodiment,the prisms have an included angle of about 89 degrees.

[0004] In another embodiment, the invention includes a back lightingdisplay device having a lighting device, a display panel, and a sheetinghaving a first side and a second side, wherein the first side includes aseries of linear prisms having peaks, and the second side includes aplurality of subwavelength structures, the subwavelength structuresbeing oriented at about 90 degrees relative to the peaks of the linearprisms.

[0005] In a further embodiment, the invention includes an opticalstructure having a first light-redirecting film having a first surfacewith a plurality of linear moth-eye structures thereon and a secondsurface with first linear prisms having peaks, the linear moth-eyestructures being about parallel or perpendicular to the peaks of thefirst linear prisms. The optical structure can also include a secondlight-redirecting film having a first surface with a plurality of linearmoth-eye structures thereon and a second surface with second linearprisms having peaks, the linear moth-eye structures being about parallelor perpendicular to the peaks of the second linear prisms.

[0006] In particular embodiments, a diffuser can be provided between thefirst light-redirecting film and the waveguide and/or a diffuser can bepositioned between the second light-redirecting film and the displaypanel.

[0007] A method of forming a light-redirecting film is also providedthat includes the steps of forming a series of linear prisms, whichinclude peaks, on a first side of a sheeting, and forming a plurality oflinear moth-eye structures on a second side of the sheeting with thelinear moth-eye structures being oriented at about 90 degrees relativeto the peaks of the linear prisms. The method can further include thesteps of forming a series of linear prisms, which also include peaks, ona first side of a second sheeting, and forming a plurality of linearmoth-eye structures on a second side of the second sheeting with thelinear moth-eye structures being oriented at about 90 degrees relativeto the peaks of the linear prisms. In one embodiment, the first andsecond sheetings are arranged such that the moth-eye structures of thefirst sheeting face the moth-eye structures of the second sheeting.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 illustrates a cross-sectional view of a backlightingsystem.

[0009]FIG. 2 illustrates a perspective view of a linear prism structure.

[0010]FIG. 3 illustrates a side view of the linear prism structure shownin FIG. 2.

[0011]FIG. 4 illustrates a cross-sectional view of a second embodimentof a back lighting system.

[0012]FIG. 5 shows a plot of reflectance as a function of angles ofincidence and polarization for a moth-eye structure with 3,300 groovesper millimeter at a light wavelength of 514.5 nm.

[0013]FIG. 6 shows a plot of reflectance as a function of angles ofincidence and polarization for a moth-eye structure with 3,300 groovesper millimeter at a light wavelength of 647.1 nm.

[0014]FIG. 7 shows a plot of reflectance for a dielectric having anindex of refraction and a smooth non-moth-eye surface.

[0015]FIG. 8 shows a theoretical plot of output from a uniform lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

[0016]FIG. 9 shows a theoretical plot of output from a uniform lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

[0017]FIG. 10 shows a theoretical plot of output from a uniform lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

[0018]FIG. 11 shows a theoretical plot of output from a uniform lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

[0019]FIG. 12 shows a theoretical plot of output from a uniform lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

[0020]FIG. 13 shows a theoretical plot of output from a uniform lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

[0021]FIG. 14 shows a theoretical plot of output from a cosine lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

[0022]FIG. 15 shows a theoretical plot of output from a cosine lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

[0023]FIG. 16 shows a theoretical plot of output from a cosine lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

[0024]FIG. 17 shows a theoretical plot of output from a cosine lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

[0025]FIG. 18 shows a theoretical plot of output from a cosine light,distribution X-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

[0026]FIG. 19 shows a theoretical plot of output from a cosine lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

[0027]FIG. 20 illustrates a side view of a subwavelength opticalmicrostructure.

[0028]FIG. 21 shows a plot of relative response versus wavelength oflight for a 0.002 inch (51 μm) thick film of polyester, 0.002 inch (51μm) thick film of polyester with one side having moth-eye structures,0.002 inch (51 μm) thick film of polyester with two sides havingmoth-eye structures, and a reference with a detector located normal tothe surface of the film.

[0029]FIG. 22 shows a plot of relative response versus wavelength oflight for a 0.002 inch (51 μm) thick film of polyester, 0.002 inch (51μm) thick film of polyester with one side having moth-eye structures,0.002 inch (51 μm) thick films of polyester with two sides havingmoth-eye structures, and a reference with a detector located at an angle30 degrees from the normal to the surface of the film.

[0030]FIG. 23 shows a plot of light transmission versus angle from thenormal of a 0.002 inch (51 μm) polyester film with and without amoth-eye structure on one side at the zero and 90 degree profile.

[0031]FIG. 24 shows a plot of color versus angle from the normal of a0.002 inch (51 μm) thick polyester film with and without moth-eyestructures on both sides observed at zero degree orientation.

[0032]FIG. 25 shows a plot of color versus angle from the normal of a0.004 inch (102 μm) thick polyester film with and without moth-eyestructures on both sides observed at zero degree X-orientation and 90degree Y-orientation.

[0033]FIG. 26 is a plot of luminance cross section versus observationangle from the normal at zero degree orientation of a film with moth-eyestructures having a period of about 0.2 μm and a height of about 0.4 μmand linear prisms with 95 degree included angle and a pitch of 0.0019inches (48 μm).

[0034]FIG. 27 is a plot of luminance cross section versus observationangle from the normal at 90 degrees orientation of a film with moth-eyestructures having a period of about 0.2 μm and a height of about 0.4 μmand linear prisms with a 95 degree included angle and a pitch of 0.0019inches (48 μm).

[0035]FIG. 28 is a plot of luminance cross section versus observationangle from the normal at zero degree orientation of a film withoutmoth-eye structures and linear prisms with 95 degree included angle anda pitch of 0.0019 inches (48 μm).

[0036]FIG. 29 is a plot of luminance cross section versus observationangle from the normal at zero degree orientation of a film withoutmoth-eye structures and linear prisms with 95 degree included angle anda pitch of 0.0019 inches (48 μm).

[0037]FIG. 30 shows a plot of light transmission versus angle from thenormal of a film with 90 degree linear prisms having a pitch of 0.002inches (51 μm) with and without moth-eye structures on the window sideof the films.

[0038] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. All percentages and parts are by weightunless otherwise indicated.

DETAILED DESCRIPTION OF THE INVENTION

[0039] With respect to the optical performance of a light-redirecting orcollimating film, it has been found that for individual AMLCD (activematrix liquid crystal display) back lighting system designs, the opticalefficiency of the particular lamp, waveguide, and diffuser system can beimproved by designing a light-redirecting film to maximize the use ofthe diffraction and refraction effects. For example, as shown in FIG. 1,a back lighting system 10 includes a light source 12 and light reflector14. Light source 12 can be a fluorescent light, incandescent light orother suitable light source. Waveguide 16, which is for directing lightout of back lighting system, can be formed of a transparent solidmaterial and is often wedge shaped. On one side of waveguide 16 iswaveguide reflector 18 formed of a specular material, such as aluminumor a coated white surface, for reflecting light back to waveguide 16.Waveguide reflector 18 can be curved or flat. Diffuser 20 is a film thatdiffuses the light from the waveguide into a substantially uniformdistribution. An example of a suitable diffuser is a randomly texturedsurface or gradient index film or engineered diffractive structure.

[0040] Above diffuser 20, first light-redirecting film 22 has moth-eyestructure 24 on a first side adjacent waveguide 16. Second side of firstlight-redirecting film 22 has prism structure 25. An optional abrasionreduction layer 26 is between first light-redirecting film 22 and secondlight-redirecting film 28. The abrasion reduction layer can have amoth-eye structure on one or two surfaces to improve performance. Secondlight-redirecting film 28 has moth-eye structure 30 on a first sideadjacent first light-redirecting film 22 and prism structure 32. Prismstructure 32 of second light-redirecting film 28 can be oriented in thesame direction as the prisms on first light-redirecting film 22.Alternatively, it may be offset by rotating the prism orientation up toabout 180 degrees. In one embodiment, the second light-redirecting filmis rotated about 90 degrees with respect to the first light-redirectingfilm to reduce moiré fringe formation and improve the uniformity of theexiting light distribution. Above the second light-redirecting film is aliquid crystal display 34 or display panel. A light-redirecting filmthat has linear prisms designed with a tilt, size, and included anglethat match the light source, waveguide, and diffuser properties providesenhanced performance. Employing linear prism arrays with included anglesthat range from 95 degrees to 120 degrees provides a light distributionthat is optimized for viewing angles of a computer screen. The includedangle is considered the top angle of a triangular linear prismstructure. A diffuser can also be positioned between the secondlight-redirecting film 28 and the display panel 34.

[0041] In alternative embodiments, the moth-eye structures 24, 30 arelinear moth-eye structures and can be oriented parallel or non-parallelto the respective linear prisms 25, 32 in a horizontal orientation. In aparticular embodiment, longitudinal axes of the moth-eye structures 24are about perpendicular to longitudinal axes of the prisms 25, whilelongitudinal axes of the moth-eye structures 30 are about perpendicularto longitudinal axes of the prisms 32. In another embodiment, themoth-eye structures 24 are about perpendicular to the prisms 25, whilethe moth-eye structures 30 are about parallel to the prisms 32. Infurther embodiments, the moth-eye structures 24 are about parallel tothe prisms 25, while the moth-eye structures 30 are about perpendicularto the prisms 32. In alternative embodiments, the moth-eye structures24, 30 are about parallel to respective prisms 25, 32. In furtherembodiments, the moth-eye structures 24 and/or 30 are orientednon-parallel and non-perpendicular to respective prisms 25, 32,depending on the desired optical properties.

[0042] When the linear moth-eye structures have a pitch near 225 nm andthe linear prism apex angle is about 90 degrees, resonance of the lightcauses a green color viewed from wide-viewing angles (angles at about 70to 80 degrees from normal) that are perpendicular to the direction ofthe linear moth-eye structures. For example, if the product isconstructed so that both films, even if they are crossed at 90 degrees,have both linear moth-eye structures oriented perpendicular to a coldcathode fluorescent light source, the color appears only in aside-to-side direction in a display where the light source is locatedacross the bottom of the display. By orienting the linear moth-eyestructures parallel to each other, the color can be controlled to be ononly one viewing axis. When the linear moth-eye structures areperpendicular to each other, the color is evident on two viewing axesthat are perpendicular to each other. This latter result is the same ashaving regularly spaced 225 nm pitch two-dimensional structures (ascompared to linear moth-eye structures) that produce color atwide-viewing angles in two axes that are perpendicular to each other.One reason for using linear moth-eye structures is to control thedirection at which the color can be viewed. When the pitch of themoth-eye structures is made to be less than about 180 nm, the resonantcolors are not present at the wide-viewing angles and the need to uselinear moth-eye structures is no longer present. In accordance withembodiments of the invention, the moth-eye structures are linear, havinga pitch between about 150 and 350 nanometers.

[0043] An example of a linear prism film is shown in a perspective viewin FIG. 2 and in a side view in FIG. 3. Linear prism film 40 has prismsurface 42 and window surface 44 and is formed of a transparentpolymeric material. Prisms 46 have sides 48 with peaks 50 and valleys52. The pitch (p) of the prisms 46 is measured from valley 52 to nextvalley 52. The pitch can be in the range of between 0.001 and 0.003inches (25 and 76 μm). The height (h) of the linear prisms is measuredby the vertical distance from the valley 52 to peak 50. The height canbe in the range of between 0.0003 and 0.0015 inches (7.6 and 38 μm). Theincluded angle (∝) is measured between the two sides that meet at peak50. The angle (∝) can range from about 60 to 120 degrees. In a preferredembodiment, the angle (∝) is in a range of between about 60 and 85degrees or between about 95 and 120 degrees. Sides 48 on each side ofthe peak 50 can be the side length (l) from valley 52 to peak 50 to forman isosceles triangle. Alternatively, the sides can have differentlengths, thereby tilting or canting the prisms. The tilting angle (β) ofthe prisms is between the optical axis 54 and a line 56 perpendicular tothe window side 44. The prisms can be tilted in the range of betweenabout −44 and +44 degrees. In one embodiment, the tilting is about sevendegrees.

[0044] In other embodiments, prism structures 22, 32 can includelenticular linear elements, such as disclosed in U.S. Pat. No.5,592,332, issued to Nishio et al. on Jan. 7, 1997.

[0045] Another embodiment of the present invention is shown in FIG. 4. Aback lighting system 100 includes a light source 102 and a lightreflector 104. Waveguide 106 can be formed of a transparent solidmaterial and can be wedge-shaped. Adjacent to the first side 108 ofwaveguide 106 is waveguide reflector 110 formed of a specular material.The reflector 110 is spaced slightly away from surface 108 to allowtotal internal reflection at surface 108 to take place. First side 108can be stepped in shape. Second side 112 of waveguide 106 is on theopposite side away from waveguide reflector 110. Second side 112 hasmoth-eye structures 114.

[0046] Above waveguide 106, first light-redirecting film 116 has firstprism structure 118 with peaks 120 pointed toward waveguide 106 andfirst moth-eye structures 122 on the window side of first prismstructure 118. Preferably, the peaks of linear prisms on firstlight-redirecting film 116 run parallel to light source 102. Above firstlight-redirecting film 116, second light-redirecting film 124 has secondmoth-eye structure 126 and second prism structure 128. Peaks 130 ofsecond prism structure 128 point away from waveguide 106. Preferably,the peaks 130 of second prism structure 128 are oriented in anon-parallel direction to peaks 120 of first prism structure 118. A morepreferred orientation is 90 degrees.

[0047] In one embodiment, the first moth-eye structures 122 are orientedat about 90 degrees relative to the peaks of the linear prisms on thefirst light-redirecting film 116. In one embodiment, angle (∝) of thelinear prisms on the first light-redirecting film 116 is about 88degrees. In another embodiment, angle (∝) is about 89 degrees.

[0048] The second moth-eye structures 126 can be oriented at about 90degrees relative to the peaks of the second light-redirecting film 124.In one embodiment, angle (∝) of the linear prisms on thelight-redirecting film 116 is about 88 degrees. In another embodiment,angle (∝) is about 89 degrees. The moth-eye structures 122, 126 areoriented at about 90 degrees relative to the linear prisms of respectivelight-redirecting films 116, 124 to minimize, and preferably eliminate,the deep blue to deep green color that is produced by light resonance,and which is visible, at wide entrance angles.

[0049] In one embodiment having the moth-eye structures 122, 126oriented 90 degrees relative to the linear prisms of respectivelight-redirecting films 116, 124, only a small amount of green colorshows at very wide-viewing angles, for example, 70 to 80 degrees fromnormal. In contrast, when the moth-eye structures 122, 126 are orientedparallel to the linear prisms of respective light-redirecting films 116,124, the off-axis color becomes a very vivid purple-blue at a viewingangle of about 45 degrees from normal. In some applications, this vividcolor may be desirable where a narrow useable viewing is needed, such asin security/privacy applications.

[0050] In the embodiment having 88 degree linear prisms onlight-redirecting films 116, 124 with moth-eye structures 122, 126oriented at about 90 degrees relative to respective linear prisms, a 4%performance increase over commercially available brightness enhancingfilms having 90 degree prisms has been realized.

[0051] In the embodiment having 89 degree linear prisms onlight-redirecting films 116, 124 with moth-eye structures 122, 126oriented at about 90 degrees relative to respective linear prisms, a 6%performance increase over commercially available brightness films having90 degree prisms has been realized.

[0052] The performance of TIR (total internally reflecting) films, oftencalled BEF (brightness enhancing film), which are used to increase thelight output from back lighting systems in AMLCD flat panel displays,can be improved by changing the tilt angle of the linear prism, thelinear prism included angle, and also the pitch of the linear prismarray. A further improvement can be made by making the film monolithicor polylithic. A monolithic film removes one material interface (at thesubstrate) and improves optical transmission. In the case of thepolylithic film, a diffuser can be incorporated into the film structure,saving the need to fabricate a separate diffuser and dependent on thedegree of collimation required.

[0053] A fine pitch of a linear corner cube prism structure providesexcellent performance as a first layer in a back lighting system if adiffuser is not used between the top smooth surface of a waveguide and aflat surface of the linear micro corner cube sheet. A fine pitch,preferably in the range of between about 0.00005 and 0.0001 inches (1.3and 2.5 μm), of the corner cube array helps to spread the refracted andretroreflected light by diffraction, creating increased diffusion ofrecycled light. In a more preferred embodiment, the pitch is about0.000075 inches (1.9 μm). The refracted and retroreflected light isspread by one to two degrees, depending on the accuracy of the linearcorner cube array dihedral angles. This spreading is then increased bydiffuse structures on the second surface of the waveguide, creating asmooth diffuse light pattern without the need of the diffuser betweenthe waveguide and linear corner cube light-redirecting sheet. Inaddition, the groove pattern in the linear corner cube array is orientedin directions that do not modulate with the diffuse dot pattern on therear of the waveguide. Therefore, moiré fringes are not created. Asurface is disclosed in U.S. Pat. No. 5,600,462, issued to Suzuki et al.on Feb. 4, 1997, the teachings of which are incorporated herein byreference, which employs a rough structure (0.004 inches, 10 μm) forperforming diffuse transmission to create a “ground glass-typediffusion.”

[0054] Also, it has been found that the addition of one or two 95 degreelinear prism sheet(s) with 0.0019 inch (48 μm) pitch above the finepitch linear corner cube sheet and with the smooth surface orientedtoward the corner cube array further enhances the brightness. The secondlinear prism sheet is oriented about 90 degrees with respect to thefirst sheet.

[0055] The materials that work well for optical microstructured filmsare ultraviolet-cured polymers bonded to a polyester substrate, whichcan have abrasion resistance which is important during handling of thelight-redirecting films. If the prism tips are damaged during handling,the resulting display can have fine lines that appear as less brightthan surrounding areas on axis and brighter than surrounding areasoff-axis. The films can be formed of suitable polymers, such aspolycarbonate. The films can be constructed from a polycarbonatematerial, acrylic, or other suitable material, such as disclosed in U.S.Pat. No. 5,396,350, issued to Beeson et al. on Mar. 7, 1995, theteachings of which are incorporated herein by reference.

[0056] An abrasion reduction sheeting, such as a thin polypropylene filmor similar material, can be placed in between the light-redirecting filmlayers to help to reduce any effect from abrasion without losingsignificant brightness. Subwavelength visible light moth-eye structurescan be used on these overleaf films to effectively eliminate Fresnelreflection light losses. The softer films do not abrade the linear prismpeaks as easily as hard films. A semi-soft substrate, such as apolyvinyl chloride film, can be used in place of the polyester substrateto make light-redirecting films and reduce abrasion. However, one mustbe careful of out-gassing and resulting surface contamination that canoccur with polyvinyl chloride.

[0057] A linear non-isosceles prism array tilted or canted in the rangeof between about −45 and +45 degrees and preferably at seven degrees,having a 95 degree included angle and a 0.0019 inch (48 μm) pitch as afirst layer and a linear isosceles prism (with zero tilt), having a 95degree included angle and a 0.0019 inch (48 μm) pitch as a second layercan significantly improve the amount of light that is directed throughan AMLCD to the angles (geometry's) desired for optimum user viewingangles. The tilt or canting of the optical axis of the first layerlinear prism array corrects the skewed direction of the lightdistribution coming from the waveguide and diffuser. A 0.0019 inch (48μm) pitch can cause diffraction spreading, which smooths the lightdistribution and maximizes the light directed toward the angles mostbeneficial or desired for a AMLCD display user. The 95 degree includedangle further optimizes the field of view of the light distribution forthe display user while still recycling light that is headed in theincorrect direction back into the display where it is used again.

[0058] Therefore, a preferred light-redirecting film combination for awedge waveguide includes a first light-redirecting film that has prismstilted to correct for the skew created by the waveguide wedge anddiffuser layers and has a prism angle designed to maximize the userfield of view plus a second light-redirecting film oriented at 90degrees to the first and with a symmetrical linear prism pattern. In thesecond light-redirecting film the prisms can be tilted uniformly in bothdirections (tilt every other prism in the opposite direction) to have aprism angle that optimizes the user field of view for this axis.

[0059] Further, performance change can be achieved by combining thediffuser into the first zone of the first light-redirecting film toeliminate one film component. However, the focusing effect of the firstsurface is lost. The diffuser can be made by employing textured filmsand casting the linear prisms onto the smooth side of the film, byrotary screen printing a diffuse layer onto the polyester tie coat priorto casting the linear prisms onto the diffuse layer (in this embodimentthe diffuse layer is sandwiched between the linear prisms and thesubstrate film), or by rotary screen printing a diffuse layer onto acarrier film and then casting linear prisms onto the diffuse layer. Theprism and diffuse layer can be made of the same material and finishcured together, by adding particles into the tie coat prior to castingthe linear prisms onto the tie coat, and by dispersing particles in thesubstrate sheet followed by casting linear prisms onto the substratesheet.

[0060] The addition of the moth-eye structure to the window side of thelight-redirecting films increases the system brightness by about 6% to8%, which is significantly brighter (by 10% to 12%) than the previouslyknown brightness-enhancing film systems with a similar pitch.

[0061] These improved results are believed to be due to a combination ofmicrostructured optical effects. The uniform white light, such as afluorescent bulb that causes this light to have a cool appearancebecause of the blue shift, has distribution coming from the diffuserthat is incident on the first layer moth-eye surface. At angles ofincidence of +/−60 degrees, 2% or less of the light is reflected at thefirst layer moth-eye to air interface. Plots of the reflectance areshown in FIGS. 5 and 6 for a subwavelength microstructure having 3,300grooves per millimeter of light having wavelengths of 514.5 nm and 647.1nm, respectively. The S line represents light perpendicular to the planeof incidence, and the P line represents light parallel to the plane ofincidence. Shown in FIG. 5, the average reflectance (linear averagebetween S and P lines) is about 0.8% at 60 degrees and shown in FIG. 6,the average reflectance is about 2% at 60 degrees. This is compared toan average of about 10% of the light that is reflected at a smoothnon-moth-eye surface at a 60 degree angle of incidence. FIG. 7 shows aplot of an average of about 10% reflectance at a 60 degree incidentangle. Also at normal incidence, a typical 4% reflectance due to asmooth surface is reduced to less than 1% with a moth-eye structure.

[0062] At angles of incidence that are greater than 75 degrees, a greenand then a blue color can be observed. The color is a result ofdiffraction scattering as the short wavelengths enter the moth-eyestructure from an angle that causes the aperture of the moth-eyeelements to become diffractive and resonate. This diffraction scatteredlight is processed by the linear prism film differently than the lightthat passes through a non-moth-eye smooth surface. In this embodiment,the green to blue light is more uniformly distributed throughout thefilm, creating a more uniform illumination. A significant amount ofgreen light is light piped by total internal reflection within the filmand is partially filtered out of the light that becomes available toilluminate an LCD panel. Different size (frequency and amplitude)moth-eye structures can be used to create different illumination effectsdepending on the light source and optical components used in theillumination system. By changing the size of the moth-eye structures inthe range from sub-wavelength to larger than wavelength scalestructures, for example, moth-eye structures having a period of betweenabout 0.15 and 10.0 micrometers, the diffraction properties of thesurface can be optimized to help smooth the resulting light distributionand improve wide angle light distribution.

[0063] After the light has passed through the first moth-eye layer, itis redirected in a collimating fashion to about 42 degrees, and the 95degree linear prism second surface of the first film layer throughrefraction collimates the light to approximately +/−30 degrees. Then thelight enters the moth-eye surface on the first surface of the secondlayer film where it is further collimated by refraction. The majority ofthe light is at +/−30 degrees from the normal as it enters the moth-eyesurface and passes through the moth-eye layer with little intensityloss. The light passes through the second layer film and is furtherredirected through refraction and recycling by the 95 degree linearprism structure. The 95 degree prism shape helps to recycle any of thelight that is still traveling at wide angles of incidence. This lighteventually emerges from the lighting system within a final +/−29 degreelight distribution in both the X and Y axes.

[0064] If two crossed 75 degree linear prism films with moth-eye smoothsurfaces are used in the illumination system, a light intensitydistribution width of +/−18 degrees and an intensity of 2.63 can beachieved. The light intensity distribution X-profile and Y-profile foruniform light through a film with 90 degree, 75 degree and 95 degreeprism films are shown. FIGS. 8 and 9 show plots of output uniform lightdistribution of the X-profile and Y-profile, respectively, for one andtwo films of 0.0019 inch (48 μm) pitch for linear prisms having a prismangle of 90 degrees. FIGS. 10 and 11 show plots of output for uniformlight distribution X-profile and Y-profile, respectively, for one andtwo films of 0.0019 inch (48 μm) pitch for linear prisms having a prismangle of 75 and 95 degrees, respectively. FIGS. 12 and 13 show plots ofoutput for uniform light distribution X-profile and Y-profile,respectively, for one and two films of 0.0019 inch (48 μm) linear prismshaving a prism angle of 75 degrees. FIGS. 14 and 15 show plots of outputcosine light distribution of the X-profile and Y-profile, respectively,for one and two films of 0.0019 inch (48 μm) pitch for linear prismshaving a prism angle of 90 degrees. FIGS. 16 and 17 show plots of outputfor cosine light distribution X-profile and Y-profile, respectively, forone and two films of 0.0019 inch (48 μm) pitch for linear prisms havinga prism angle of 75 and 95 degrees respectively. FIGS. 18 and 19 showplots of output for cosine light distribution X-profile and Y-profile,respectively, for one and two films of 0.0019 inch (48 μm) linear prismshaving a prism angle of 75 degrees. Additional optimization of theangles allows a near +/−10 degree intensity distribution. Onedisadvantage with this configuration is an approximate +/−2.0 degreevoid that appears at the center of the light intensity distribution.This effect is visible in FIGS. 12 and 13. Slight curvature orpositive-negative canting in the prism facets can reduce this void.

[0065] The application of a moth-eye structure to the smooth surface ofthe linear prism films significantly improves the light-redirectingcapability of the films by increasing light throughput at the moth-eyestructured surface and redirecting wide incident angle light rays.Diffraction effects also play a significant role in the improvedperformance of the system. The resulting color of the backlight assemblyis warmer in appearance than the same assembly without the addition ofthe moth-eye structures. This color shift can have a beneficial effecton the contrast within the final back light display.

[0066] As shown in FIG. 20, the moth-eye structure applied preferablyhas an amplitude (A) of about 0.4 micrometers and a period (P) of lessthan about 0.2 micrometers. The structure is sinusoidal in appearanceand can provide a deep green to deep blue color when viewed at grazingangles of incidence. Preferably, the amplitude is about three times theperiod to provide a three to one aspect ratio.

[0067]FIGS. 21 and 22 show a plot of the improvement in transmission bywavelength for 0.002 inch (51 μm) thick PET having moth-eye structureson one side and having moth-eye structures on both sides at zero degreesand at 30 degree angles from the normal, respectively. The moth-eyestructures have a period of about 0.2 μm and a height of about 0.4 μm.The reference is a uniform light distribution coming from a diffuserpositioned above the waveguide. FIG. 23 shows a plot of the improvementin transmission by angle from the normal for 0.002 inch (51 μm) PET withmoth-eye structures. In this figure, the fluorescent tube light bulb isat a +80 degree position for the 90 degree orientation. FIG. 24 shows aplot of the color shift which occurs for 0.002 inch (51 μm) thick PETwith moth-eye structures on one side and with moth-eye structures onboth sides. FIG. 25 shows a plot of the color shift that occurs for0.004 inch (102 μm) thick PET with 95 degree linear prisms on the sideaway from the diffuser and with and without moth eye on the side closeto the diffuser. For all measurements, the samples were placed on top ofthe diffuser in a standard LCD back light assembly and the PhotonResearch detector, Model PR650, was supported eighteen inches (45.7 cm)above the part surface.

[0068] The moth-eye structure provides anti-reflection properties to thepreviously smooth light entrance surface of the substrate even atentrance angles that are near grazing incidence. The moth-eye structureis more effective than standard thin film anti-reflection coatings atwide angles of incidence, especially angles of incidence beyond 30degrees up to 80 degrees. This characteristic causes many types ofoptical microstructure films, including linear prism films, to processlight very differently than the standard linear prism light-redirectingfilms that have smooth entrance surfaces with or without standardanti-reflection thin film (vacuum deposited or liquid applied) coatings.The addition of the moth-eye structures helps to more efficientlyrecycle light and also redirects the normally reflected grazing angleincidence rays into the optical microstructure (such as linear prisms)sheet where the rays are refracted, reflected or retroreflecteddepending on the respective angles of incidence. This moth-eyeimprovement concept can be added to many types of brightness enhancementfilms (BEF). An advantage is that functional optical microstructures canbe applied to both sides of a film or substrate.

[0069] A moth-eye anti-reflection surface is one in which the reflectionof light is reduced by the presence of a regular array of smallprotuberances covering the surface. The spacing of the protuberances isless than the wavelength of light for which anti-reflection is sought. Amoth-eye surface can be understood in terms of a surface layer in whichthe refractive index varies gradually from unity to that of the bulkmaterial. Without such a layer the Fresnel reflection coefficient at aninterface of two media is equal to ((n₁−n₂)/(n₁+n₂))², where n₁ and n₂are the refractive indices of the media. However, if there is a gradualchange of index, net reflectance can be regarded as the result of aninfinite series of reflections at each incremental change in index.Since each reflection comes from a different depth from the surface,each has a different phase. If a transition takes place over an opticaldistance of λ/2, all phases are present, there is destructiveinterference and the reflectance falls to zero.

[0070] When the height of the protuberance (h) is significantly lessthan the wavelength (λ), the interface appears relatively sharp and thereflectance is essentially that of a discontinuous boundary. As theratio of h/λ increases, the reflectance decreases to a minimum value atabout h/λ=0.4. Further increases in h/λ show a series of successivemaxima and minima, but the value does not again approach that of a sharpinterface. The details of the curve shown in FIG. 20 vary depending onthe profile of the change of the index of refraction, but if thethickness is of the order of half a wavelength or more the reflectanceis considerably reduced. The spacing of the protuberances should besufficiently fine to avoid losses by diffraction. Preferably, it shouldbe less than the shortest wavelength involved divided by the refractiveindex of the material.

[0071] It is important that the spacing d between the peaks of theprotuberances on the moth-eye surface is sufficiently small that thearray cannot be resolved by incident light. If this is not the case, thearray can act as a diffraction grating and, although there may well be areduction in the specular reflection (zero order), the light is simplyredistributed into the diffracted orders. In other words, we requirethat d<λ for normal incidence and d<λ/2 for oblique incidence if forreflection only, and that d<λ/2n in the case of transmission wherediffraction inside the material is suppressed.

[0072] For a given moth-eye surface, where the height of theprotuberances is h and the spacing is d, the reflectance is expected tobe very low for wavelengths less than about 2.5 h and greater than d atnormal incidence, and for wavelengths greater than 2d for obliqueincidence. Preferably, the spacing is as close as possible, and thedepth as great as possible, in order to give the widest possiblebandwidth. For example, a h/d ratio is preferably about three.

[0073] The moth-eye effect should not be confused with that of reducingthe specular reflectance by roughening. Roughness merely redistributesthe reflected light as diffuse scattering and degrades the transmittedwavefront. With the moth-eye structure, there is no increase in diffusescattering, the transmitted wavefront is not degraded, and the reductionin reflection gives rise to a corresponding increase in transmission.

[0074] The moth-eye structure has many advantages. There is no extracoating process necessary. The structure can be transferred to the sheetby a pressure molding process, such as with a Fresnel structure. Thereflection reduction does not depend on the wavelength. There is only alower limit (on the ultraviolet side of the spectrum) set by thestructure period. If the wavelength is too small compared to the period,the light is diffracted. In regard to angular dependence, withconventional anti-reflective coatings, the transmission curve shiftswith the light incidence angle. With the moth-eye structure, thecritical wavelength for diffraction shifts to higher values, but thereare no changes above this wavelength. Another advantage for moth-eyestructures is that there are no adhesion problems between lens andgradient layer because it can be one bulk material. From a high incidentangle, the surfaces can appear blue or violet.

[0075] To form a moth-eye structure, the structure is first produced ona photoresist-covered glass substrate by a holographic exposure using anultraviolet laser. A suitable device is available from HolographicLithography Systems of Bedford, Mass. 01730. An example of a method isdisclosed in U.S. Pat. No. 4,013,465, issued to Clapham et al. on Mar.22, 1977, the teachings of which are incorporated herein by reference.This method is sensitive to any changes in the environment, such astemperature and dust, and care must taken. The structure is thentransferred to a nickel shim by an electroforming process. In apreferred embodiment, the shims are about 300 micrometers thick or less.

[0076] The moth-eye structures can be made one dimensional in a gratingtype pattern. In this embodiment, the structure has a nearly rectangularprofile, which means they have no gradient layers, but more of a onelayer anti-reflective coating with a lowered refractive index in thestructure region. Control of the grating depth is important as iscontrol of thickness for the evaporated layers. Control of depth andthickness is achieved by maintaining uniformity of beam exposure,substrate flatness, and exposure time.

[0077] A two-dimensional structure is formed by two exposures with alinear sinus-grid, turned by 90 degrees for the second exposure. A thirdtype of structure is formed by three exposures with turns of 60 degreesto provide a hexagonal or honeycomb shape.

[0078] When measured at a four inch (10.2 cm) distance from the displayPhoton Research Model No. PR650, the results with two 95 degree linearprism films each having a moth-eye structure on the previously smoothside show about the same brightness on axis as two 90 degree BEF films,a large improvement in brightness off axis in both vertical andhorizontal axis and a warmer color to the light emerging from thedisplay. In FIGS. 26, 27, 28 and 29, the total integrated lightintensity for the 95 degree prisms with moth-eye structure films is6,686.8 Im/m² with a maximum of 4,460 cd/m² and a minimum of 554.0cd/m². For the 90 degree prisms without moth-eye structure films, theintegrated light intensity is 5,698.8 Im/m² with a maximum of 4,685.0cd/M² and a minimum of 295.9 cd/m².

[0079] Through analysis and experimental results, a preferred embodimentincludes a 75 degree linear prism film that can be used as the firstlayer above a uniform light output diffuser to collimate the light toabout a +/−30 degree angle. The prism grooves in this first layer areoriented parallel to the light source that illuminates the waveguidethat is below the diffuser. On top of this film can be a 95 degreelinear prism film that is oriented at 90 degrees with respect to the 75degree film to collimate the light to about +/−25 degrees with a smallpercentage of the light at +/−30 degrees, as shown in FIGS. 10, 11, 16and 17. The final intensity of the collimated light is excellent andcomparable to results obtained with two crossed 90 degrees BEF films, asshown in FIGS. 8, 9, 14 and 15. These 90 degree BEF films do not allowfor the recycled component (Note that there is no recycled light withthe 75 degree film) but, allowing for the recycled light, the peakintensities become 2.15 for the 75 degree plus 95 degreelight-redirecting films and 2.06 for the BEF films. The prism apex angleof the second 95 degree film can be increased to about 100 degrees ifthe spread in the collimated light beam is too narrow.

[0080]FIG. 30 shows a comparative plot of light transmission versusangle from the normal of a film with 90 degree linear prisms having apitch of 0.002 inches (51 μm) with moth-eye structures on the windowside of the film and a film with 90 degree linear prisms having a pitchof 0.002 inches (51 μm) without moth-eye structures on the side of thefilm. The comparative plot shows a substantial improvement intransmission, particularly at zero degrees, when employing a moth-eyestructure on the window side of the film as compared to a similar filmwithout a moth-eye structure.

[0081] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A waveguide for use in a back lighting displaydevice, the waveguide having a moth-eye structured surface thereon. 2.The waveguide of claim 1, wherein the waveguide is wedge-shaped.
 3. Thewaveguide of claim 1, wherein the waveguide is configured to beimplemented in a back lighting system.
 4. A back lighting system,comprising; a lighting device; a display panel; and a waveguide forredirecting light generated by the lighting device toward the displaypanel, the waveguide including a plurality of moth-eye structures on atleast one surface thereof.
 5. The back lighting system of claim 4,further comprising a first light-redirecting film positioned between thewaveguide and the display panel.
 6. The back lighting system of claim 5,wherein the first light-redirecting film includes a plurality ofmoth-eye structures on a first side of the first light-redirecting film.7. The back lighting system of claim 6, wherein the moth-eye structuresface the waveguide.
 8. The back lighting system of claim 6, wherein themoth-eye structures face the display panel.
 9. The back lighting systemof claim 6, wherein the first light-redirecting film includes aplurality of linear prisms on a second side of the firstlight-redirecting film, the linear prisms facing the display panel. 10.The back lighting system of claim 5, further comprising a secondlight-redirecting film positioned between the first light-redirectingfilm and the display panel.
 11. The back lighting system of claim 10,further comprising a diffuser positioned between the secondlight-redirecting film and the display panel.
 12. The back lightingsystem of claim 10, wherein the second light-redirecting film includes aplurality of moth-eye structures on a first side of the secondlight-redirecting film, the moth-eye structures facing the firstlight-redirecting film.
 13. The back lighting system of claim 12,wherein the second light-redirecting film includes a plurality of linearprisms on a second side of the second light-redirecting film, the linearprisms facing the display device.
 14. The back lighting system of claim5, further comprising a diffuser positioned between the firstlight-redirecting film and the waveguide.
 15. The back lighting systemof claim 5, further comprising a diffuser positioned between the firstlight-redirecting film and the display panel.
 16. The back lightingsystem of claim 4, further comprising a reflector for redirecting lighttoward the waveguide.
 17. A light-redirecting film comprising a sheetinghaving a plurality of prisms on a first side of the film, and aplurality of moth-eye structures on a second side of the film.
 18. Thelight-redirecting film of claim 17, wherein the prisms include linearprisms.
 19. The light-redirecting film of claim 18, wherein the moth-eyestructures include linear moth-eye structures.
 20. An optical structure,comprising: a) a first light-redirecting film having a first surfacewith a plurality of linear moth-eye structures thereon and a secondsurface with first linear prisms having peaks; and b) a secondlight-redirecting film having a first surface with a plurality of linearmoth-eye structures thereon and a second surface with second linearprisms having peaks.
 21. The optical structure of claim 20, furthercomprising an abrasion reduction layer positioned between the first andsecond light-redirecting films.
 22. The optical structure of claim 20,further comprising a waveguide positioned adjacent to the firstlight-redirecting film, the waveguide including a moth-eye structuredsurface.
 23. The optical structure of claim 22, further comprising adiffuser positioned between the first light-redirecting film and thewaveguide.
 24. The optical structure of claim 20, wherein the linearmoth-eye structures of the first light-redirecting film are oriented atabout ninety degrees relative to the peaks of the first linear prisms.25. The optical structure of claim 20, wherein the linear moth-eyestructures of the second light-redirecting film are oriented at aboutninety degrees relative to the peaks of the second linear prisms. 26.The optical structure of claim 20, further comprising a diffuserpositioned between the second light-redirecting layer and the displaypanel.
 27. The optical structure of claim 20, wherein the linearmoth-eye structures of the second film are substantially parallel to thepeaks of the linear prisms of the first film.
 28. The optical structureof claim 20, wherein the linear moth-eye structures of the second filmare substantially perpendicular to the peaks of the linear prisms of thefirst film.
 29. The optical structure of claim 20, wherein the linearmoth-eye structures of the first light-redirecting film aresubstantially parallel to the peaks of the first linear prisms.
 30. Theoptical structure of claim 20, wherein the linear moth-eye structures ofthe second light-redirecting film are substantially parallel to thepeaks of the second linear prisms.
 31. The optical structure of claim20, wherein the peaks of the first linear prisms are substantiallyperpendicular to the peaks of the second linear prisms.
 32. The opticalstructure of claim 20, wherein the linear moth-eye structures have apitch between about 150 and 350 nanometers.
 33. An optical structure,comprising: a) a first light-redirecting film having a plurality oflinear prisms on one side and a moth-eye structured surface on anopposing side; and b) a second light-redirecting film having a pluralityof linear prisms on one side and a moth-eye structured surface on anopposing side.
 34. The optical structure of claim 33, wherein themoth-eye structured surface of the first and second films includeslinear moth-eye structures, the moth-eye structures of the first filmbeing about perpendicular to the linear prisms of the first film and themoth-eye structures of the second film being about perpendicular to thelinear prisms of the second film.
 35. The optical structure of claim 33,wherein the moth-eye structured surface of the first and second filmsincludes linear moth-eye structures, the moth-eye structures of thefirst film being about parallel to the linear prisms of the first filmand the moth-eye structures of the second film being about perpendicularto the linear prisms of the second film.
 36. The optical structure ofclaim 33, wherein the moth-eye structured surface of the first andsecond films includes linear moth-eye structures, the moth-eyestructures of the first film being about perpendicular to the linearprisms of the first film and the moth-eye structures of the second filmbeing about parallel to the linear prisms of the second film.
 37. Theoptical structure of claim 33, wherein the moth-eye structured surfaceof the first and second films includes linear moth-eye structures, themoth-eye structures of the first film being about parallel to the linearprisms of the first film and the moth-eye structures of the second filmbeing about parallel to the linear prisms of the second film.